Due to continuing technological innovations in the field of semiconductor fabrication, integrated circuit chips are being developed with larger scale of integration and higher device density, as well as lower power consumption and higher operating speeds. In general, integrated circuits are manufactured using FEOL (front-end-of-line) processing technologies to form discrete semiconductor devices within the surface of a silicon wafer followed by BEOL (back-end-of-line) processing techniques to form a multi-level metallurgical interconnection network over the semiconductor devices to provide the wiring and contacts between the semiconductor devices to create the desired circuits. When semiconductor integrated circuits are designed based on sub-micron dimensions and beyond, it is very important that tight dimensional control is achieved since slight variations in processing conditions can generate significant dimensional deviations of the patterned features or other electrical defects. In this regard, semiconductor wafers are typically inspected at various stages/levels of development to ensure quality control and detect and eliminate critical, yield-limiting defects.
For example, electron beam inspection is a common technique that is employed using an SEM (scanning electron microscope) to detect electrical and physical defects on a semiconductor wafer through voltage contrast inspection of a secondary electron image. In general, electron beam voltage contrast inspection involves scanning a target region of the wafer with a focused electron beam emitted by an SEM. The electron beam irradiates the target region causing the emission of secondary electrons and a secondary electron detector measures the intensity of the secondary electron emission along the scan path of the electron beam. As a region is scanned, electrons from the electron beam induce surface voltages that vary over the scanned region due to differential charge accumulation of the irradiated features. Voltage contrast inspection operates on the principle that differences in the induced surface voltages over a scanned region will cause differences in secondary electron emission intensities.
In general, for a given feature, the intensity of secondary electron emission will vary depending on, e.g., the landing energy of the beam electrons (primary electrons) and material composition of the feature. For a given material, a secondary electron yield δ is a measure of a ratio of secondary electron emission to impinging primary electrons as a function of landing energy (eV), i.e.,
  δ  =                    Secondary        ⁢                                  ⁢        Electrons        ⁢                                  ⁢        Emitted                    Electrons        ⁢                                  ⁢        In              .  Different materials irradiated by an electron beams tuned to a specific landing energy will emit different intensities of secondary electrons. The different features within the scanned target region will be displayed in an SEM image with different grayscale shades depending on the intensity of secondary electron emission. The irradiated features having a higher intensity of secondary electron emission may be displayed brighter in an SEM image than those irradiated features having a lower intensity of secondary electron emission.
One common application of e-beam inspection is to perform in-line voltage contrast inspection of conductive features such as interconnects and through-vias to detect electrical defects such as electrical shorts, electrical opens, or resistive shorts/opens, etc. For conductive features, electrical defects can be detected as voltage contrast defects due to charging differences between defective structures and non-defective structures. For instance, FIG. 1 schematically illustrates a conventional electron beam inspection process for voltage contrast inspection of metallic contacts/vias features of a CMOS integrated circuit. In particular, FIG. 1 schematically illustrates a conventional voltage contrast inspection process to detect electrical defects for through-via contacts in a target region (10) of a semiconductor wafer substrate having a first active region (11) and a second active region (12) of a CMOS integrated circuit. The first active region (11) may be a PMOS region comprising an n-well region (11a) and a p+ diffusion region (11b), and the second active region (12) may be an NMOS region comprising a p-well region (12a) and an n+ diffusion region (12b).
A plurality of contacts (13) and (14) are formed on the p+ diffusion region (11b) in the first region (11) and a plurality of contacts (15) and (16) are formed on the n+ diffusion region (12b) in the second region (12). The contacts (13˜16) are formed in an insulating layer (not shown). The contacts (13) and (14) may be metal contacts (e.g., tungsten plugs) that are formed as part of a CMOS integrated circuit to connect the p+ diffusion region (11b) (source/drain) of a PMOS transistor to first level metallization. Similarly, the contacts (15) and (16) may be metal contacts (e.g., tungsten plugs) that are formed to connect the n+ diffusion region (12b) (source/drain) of an NMOS transistor to first level metallization. In the illustrative embodiment of FIG. 1, the contacts (14) and (16) are depicted as defective structures having electrical open defects (14a) and (16a), respectively, while the contacts (13) and (15) are depicted as non-defective structures. The electrical open defects can be readily detected using e-beam inspection due to the sharp voltage contrast differences that would result from differential charging between the defective and non-defective contacts.
In particular, during an electron beam inspection, the semiconductor wafer would be electrically connected to SEM ground (G) or some reference voltage source to control charge build up on the scanned region due to electron beam irradiation. When irradiated by the electron beam, charge accumulation on non-defective contacts (13), (15) would be discharged to SEM ground G through the wafer substrate while charge accumulation on the defective contacts (14), (16) would not foe discharged to SEM ground G due to the open defects (14a), (16a), resulting in differential charge accumulation and thus, different intensities of secondary electron emission. When examined with an SEM in secondary electron emission mode, the differentially charged non-defective contacts (13), (15) and defective contacts (14), (16) would appear in the SEM image with different intensities.
In the conventional embodiment of FIG. 1, two electron beam scans are required to adequately detect voltage contrast defects of contacts in the first and second regions (11) and (12) due to the presence of semiconductor p/n junctions (11b/11a) and (12a/12b) of the respective first and second active regions (11) and (12). For a given scan, the charge accumulation on the contacts (13˜16) may be either positive or negative, depending on the secondary electron yield  of the material forming the contacts (13˜16). In this regard, for a given scan, the p/n junction of one of the first or second regions (11) and (12) can be reversed biased, thereby preventing charge build up on contacts within the given region (11) or (12) from being discharged to SEM ground G. Consequently, there would be very little or no charge differential between defective and non-defective contacts and, thus, no discernable contrast differences in an SEM image between defective and non-defective contacts in the given region.
By way of specific example, assume for illustrative purposes that the contacts (13-16) are formed of tungsten. FIG. 8 illustrates an electron yield curve for tungsten. FIG. 8 generally illustrates that the secondary electron yield δ for tungsten is greater than 1 for a range R_1 of low landing energies and less than 1 for a range R_2 of high landing energies. In this regard, electrically isolated tungsten contacts would be positively charged when irradiated with a low landing energy electron beam, while electrically isolated tungsten contacts would be negatively charged when irradiated with a high landing energy electron beam. FIG. 1 illustrates a voltage contrast inspection process to inspect the contacts (13) and (14) in the first region (11) using a low landing energy electron beam to irradiate the target region (10). The beam electrons (indicated by solid arrows) impinging on the top surface of the contacts (13, 14, 15, 16) would cause a positive charge to accumulate in the contacts (13, 14, 15, and 16).
In the first region (11), positive charge accumulation in the non-defective contact (13) causes the p/n junction (11b/11a) to be forward biased, which allows the positive charge to be discharged to the substrate ground G through the first substrate region (11). On the other hand, the positive charge that accumulates in the defective contact (14) is not discharged to substrate ground G due to the open defect (14a). The induced surface voltage on the defective contact (14) from positive charge accumulation operates to modulate the intensity of secondary electron emission by creating potential barriers reducing secondary electron emission. In particular, the positive charge build up on the defective contact (14) decreases the intensity of the secondary electron emission as compared to the non-defective contact (13). Therefore, when the potential state of the scanned region (11) is acquired as a voltage contrast image, the lower potential non-defective contact (13) in the first region (11) might be displayed as bright (i.e., the intensity of the secondary electron emission is high) whereas the higher potential defective contact (14) is displayed as dart (i.e., lower intensity secondary electron emission).
However, in the second region (12), the positive charge that accumulates in the non-defective contact (15) causes the p/n junction (12a/12b) to be reversed biased, which prevents the accumulated charge on the contact (15) from being discharged to substrate ground G. Consequently, for low landing energies, positive charge accumulates on both the non-defective contact (15) and defective contact (16) in the second region (12), which causes the voltage contrast between such contacts to be dramatically reduced (e.g., contacts (15) and (16) are positively charged and will appear as dark regions in the SEM image). In this regard, although the contact (15) is not defective, the contact (15) can appear in the SEM image as a dark region providing a false indication of being defective.
Therefore, in the conventional process of FIG. 1, a second scan at a high landing energy would be needed to adequately detect voltage contrast defects in the second region (12). As shown in FIG. 8, the electron yield δ for tungsten at high landing energies is less than 1 such that negative charge would accumulate in the contacts (13, 14, 15 and 16). In the second region (12), the negative charge that accumulates in the non-defective contact (15) would cause the p/n junction (12a/12b) to be forward biased, which allows the negative charge to be discharged to substrate ground G. On the other hand, the negative charge that accumulates in the defective contact (16) would not be discharged to substrate ground G due to the open defect (16a). In this regard, the charge differential between the contacts (15) and (16) would result in differences in secondary electron emission and thereby resulting in voltage contrast differences suitable for voltage contrast defect detection.
However, in the first region (11), the negative charge that accumulates on the non-defective contact (13) would cause the p/n junction (11b/11a) to be reversed biased, which prevents the negative charge from being discharged to the substrate ground G. Thus, negative charge build up on both contacts (13) and (14) in the first region (11) would dramatically reduce the voltage contrast differences between such contacts and make the non-defective contact (13) to appear defective (as compared to the non-defective contact (15) in the second region (12)).
Therefore, in the conventional embodiment of FIG. 1, two separate electron beams scans at different landing energies are required (low and high) to enable voltage contrast inspection of electrical open defects of the contacts in the first and second regions (11) and (12). This is undesirable for several reasons. For instance, the need to perform two e-beam scans at different landing energies increases the inspection time and analysis and cost. Moreover, performing a second scan using a high landing energy beam is problematic in that the high energy electron beam can lead to significant charge accumulation of inspected features, which can cause distortions in an SEM image in way that decreases the efficacy of voltage contrast inspection. Moreover, the high-levels of accumulated charges that results from high energy electron beam scans can cause damage to certain semiconductor integrated circuit elements, thus lowering yield.